1. Field
This disclosure relates generally to porous covalent organic frameworks (COF) supported noble metal-free nanoparticles of general formula (I) as electro catalyst for water splitting system and to the process for preparation thereof.
2. Description of Related Art
Water is the most important and abundantly available chemical resource for producing oxygen and hydrogen by electrocatalytic reduction; 2H2O(l)→O2(g)+4H*(aq)+4e−. Electro catalysts with overpotential less than 300 mV at a current density of 10 mA/cm2 have been benchmarked as promising electro catalysts. Noble metal based nanoparticle systems such as use of RuO2 and IrO2 are known to perform as superior electro catalysts in a reaction for artificially acquiring electrons from water such as in oxygen generation reactions (OER). The noble metal catalysts, however, find limited application on a large scale due to high cost of the precious metal.
Some water splitting catalysts comprise:
at least one 3d-block transition metal element selected from manganese, iron, cobalt, nickel, and copper or a compound containing the 3d-block transition metal element; and
a base and/or a carbonate having a pKa of 8 or less.
The catalyst is preferably beta manganese dioxide and the base is selected from nitrogen-containing heterocyclic compounds or collidine. To achieve optimum catalytic activity, it is essential to perform the water oxidation reaction within a neutral range since Mn(III) formed as an intermediate during oxidation of water is found to reduce the catalytic activity.
As an alternative to the noble metal based electro catalyst systems, the binary or ternary oxides or hydroxides of metals such as Ni, Co, Fe or Mn were developed as promising catalysts. Fe—Co—Ni metal oxide film systems exhibit an overpotential of ˜200 mV. However, these noble metal-free electro catalysts need further improvement in terms of their kinetics and their overpotential towards oxygen evolution reaction (OER).
Minimizing the amount of metal concentration in the working catalyst by suspending in organic supports manifests directly the Turnover numbers and frequencies, which translate to cheaper costs. Hence, improvements were made in the art by dispersing metal nanoparticles, other than the noble metals, randomly on to conducting carbon rich supports such as graphene or carbon nanotubes (CNT). Redox active nanoparticles suspended on a conducting support (graphene, CNT) have been found to be effective systems for electro catalytic water splitting. The use of such composite catalysts help in reducing the activation energy associated with the anodic oxygen evolution reaction, 2H2O(l)→O2(g)+4H*(aq)+4e− which is quantified in terms of overpotential (η), the excess potential required over the thermodynamic limit of oxygen evolution from water i.e. of 1.23V.
The electrolytic splitting of water may be performed using a carbon-supported manganese oxide (MnOx) composite under neutral electrolyte conditions. The carbon nanotubes used as a carbon support are functionalized using oxidation reagents selected from nitric acid, sulphuric acid, potassium chlorate, persulfate or hydrogen peroxide to homogeneously disperse the nanoparticles on to the support. The composite is characterized by a 190 mV lower overpotential measured at 2 mA cm−2 when comprised in an OER-electrode for the electrolytic splitting of water under neutral electrolyte conditions. The MnOx composite is preferably a carbon-nanostructured material, e.g., a carbon-nanofiber and/or a carbon-nanotube (CNT), in to which is dispersed and deposited manganese oxide nanoparticles. The catalytic activity as well as stability is found to vary with the electronic differences such as oxidation states of manganese, Mn—O bonding and bond distances, crystallinity or variations in surface area and dispersion on the support or in support interactions between the MnOx materials.
A need still exists in the art to provide a porous, polymeric material which is stable, rigid, has exceptional thermal stabilities and can offer organic backbone or support for the incorporation of nanoparticles that can potentially tune the metal-support interactions to achieve maximum catalytic activity in water splitting reactions.
Covalent organic frameworks (COFs) due to their ordered structure, porosity and high surface area serve as crystalline organic supports and find use in several applications. The organic backbone can be manipulated to introduce specific functional groups and thereby specific chemical characteristics.
Some COFs comprise a plurality of amine subunits selected from the group consisting of diamines, triamines, and tetraamines; and a plurality of aldehyde subunits selected from the group consisting of dialdehydes, trialdehydes, and tetraaldehydes, where each di-, tri-, or tetraamine subunit is bonded with at least one aldehyde subunit by an imine bond. The iminic nitrogens are located at the cavities of the COFs and can be further functionalized with a metal atom selected from the group consisting of Mn, Fe, Co, Ni, Ru, Pt, Pd, Rh, Ir, Au, Nd, Eu and mixtures thereof, which confers to the material added chemical, electronic, magnetic, optical and redox features. The COF's are used in devices such as solar cells, flexible displays, lighting devices, sensors, photoreceptors, batteries, capacitators, gas storage devices and gas separation devices.
Other useful catalysts include spiral ultrathin-nanosheets with overgrown edges (SUNOE) of NiFe, CoNi and CoFe bimetallic hydroxides. Such SUNOE catalysts show good performance for the oxygen evolution reaction (OER) in the electrolysis of water, and the lowest onset potential was 1.45 V (vs. RHE) (the lowest potential when the current density reached 10 mA cm−2 was 1.51 V (vs. RHE)). Also, nickel nitride (Ni3N) may be used on nickel (Ni) foams for electrocatalytic applications. The Ni3N/Ni-foam exhibits extremely low overpotential (˜50 mV), high current density and excellent stability for the hydrogen evolution reaction (HER) in alkaline solution. Such modified foams show enhanced activity in the oxygen evolution (OER) and reduction (ORR) reaction compared to original Ni-foam.
The present inventors have demonstrated the use of COF with metal nanoparticles with improved interactions by incorporating N-rich heterocycles in multi-fold Heck reactions, aqueous phase C—C couplings and CO oxidation reactions. The π-system of sp2 hybridized nitrogen interacted with the nanoparticles while still allowing for a planar framework that can stack to form a crystalline 3D structure with large mesopores, a feature specific to COF.
In light of the above, the present inventors observed that there is a scope to provide COF's with suspended redox active non-precious nanoparticles with improved metal-support electronic interactions which allow the splitting of water while achieving a stable, improved catalytic activity and a low overpotential in an oxygen evolution reaction (OER).
The foregoing objects and advantages of the invention are illustrative of those that can be achieved by the various exemplary embodiments and are not intended to be exhaustive or limiting of the possible advantages which can be realized. Thus, these and other objects and advantages of the various exemplary embodiments will be apparent from the description herein or can be learned from practicing the various exemplary embodiments, both as embodied herein or as modified in view of any variation that may be apparent to those skilled in the art. Accordingly, the present invention resides in the novel methods, arrangements, combinations, and improvements herein shown and described in various exemplary embodiments.